Development of resistance in calves to nymphs of Rhipicephalus appendiculatus (Acarina:Ixodidae) during test feeds.

A test of host resistance to infestation by the tick Rhipicephalus appendiculatus was repeated 5 times, at weekly intervals, on 2 initially susceptible calves. One hundred nymphs (NN) were used each time. Pairs of tick-susceptible rabbits served as controls. The engorged NN were weighed, then allowed to moult individually, and the sex of each was noted after emergence of the adult. During the course of the experiment there was a significant reduction in mean engorged weight (especially in the pre-male NN) and also in the length of the feeding period, and a downward trend in the percentage of NN which engorged. There was a significant increase in the percentage of engorged NN that moulted. Pre-male NN were significantly heavier than pre-female NN after feeding on susceptible hosts, but though mean feeding times were the same, they took 1-2 days longer to moult than pre-females at 28 degrees C. It was concluded that the test itself could induce resistance if applied to fully susceptible calves.